Strontium oxide (SrO) is a promising material for CO₂ capture through a reversible cycle of carbonation and calcination, where SrO reacts with CO₂ to form strontium carbonate (SrCO₃) and can be regenerated by calcination. In the presence of moisture, SrO forms strontium hydroxide and its hydrates (Sr(OH)₂×nH₂O). This study employs density functional theory (DFT) to investigate the underlying mechanism of these processes on various crystal surfaces of SrO, Sr(OH)₂, Sr(OH)₂·H₂O, and Sr(OH)₂·8H₂O. The interaction of CO₂ with these surfaces leads to carbonate formation via electron transfer, with notable differences in CO₂ orientation and bond characteristics between SrO and the hydroxylated surfaces. The hydration of SrO increases the CO₂ surface adsorption energy with the exception of the monohydrate. CO₂ adsorption reaction on the Sr(OH)₂·H₂O surface is more thermodynamically favorable than on the anhydrate and octahydrate surfaces. Reaction pathway analysis reveals that bicarbonate formation is preferred on the anhydrate and octahydrate surfaces, while carbonate formation is favored on the SrO and monohydrate surfaces. The CO₂ removal reaction from the SrCO₃ surface is also investigated. This study provides valuable insights into the fundamental mechanism for CO₂ capture using SrO and its hydrated forms.